1. FIELD OF THE INVENTION
This invention relates generally to an ultrasonic length measuring system and more particularly, to an ultrasonic length measuring system which is extremely accurate and reliable in operation and which is especially adapted to measure the length of stressed bolts in situ.
2. PRIOR ART
It is generally well-known that there are numerous applications in which bolts as long as twelve feet and having cross-sectional diameters as large as six inches or more, are used in extremely high stress conditions to secure one part to another over a period of many years. For example, such large bolts may be used to secure the respective flanges of very large pipes or of valves in nuclear and refinery applications. Similarly, large bolts may be used to secure portions of a large vessel such as a nuclear reactor chamber or chemically reactive vessel where it is important that the bolts remain structurally viable despite the application of extremely high stresses over long periods of time. It is generally well-known that when such bolts are installed and tightened, it is important that the bolt's percentage of yield strength used be known to be sure that the bolt remains structurally viable. One method of determining the percentage of yield strength used is by measuring the bolt elongation resulting from the induced stress. For example, the best available high strength bolt material which has a yield strength of 240 Kpsi., will elongate 3.2 thousandths of an inch per inch of grip length at 40% of yield strength and 4.8 thousandths of an inch per inch of grip length at 60% of yield strength. This 40% to 60% yield strength range is considered generally acceptable in terms of structural viability for a tightened bolt. Thus, by measuring the elongation of a bolt as it is being tightened, the structural integrity of the bolt can be assessed and the optimum elongation can be obtained for a particular bolt material and length.
One prior art technique for measuring the actual length, and thus the elongation of bolts under stress, incorporates ultrasonic thickness measuring devices. Such devices include a transducer that is placed at one end of the bolt and an electronic gauge that responds to the reflections induced by the transducer. The gauge determines the distance between the first and second ends of the bolt on the basis of the time between reflections and the assumed velocity of the acoustic energy transmitted therethrough. Then, by comparing the measured length of the bolt under stress with a record of the actual length of the bolt unstressed, such as prior to its installation, an inspector can determine the change in length resulting from the continuous application of high stress and thereby ascertain whether or not the bolt is still of reliable structural integrity.
It will be recognized by those familiar with the art to which the present invention pertains, that the accuracy and resolution of the stressed bolt length measurement must be sufficiently high so that the change of bolt length computed therefrom provides a reliable indication of the actual elongation of the stressed bolt subject to measurement. Unfortunately, there are a number of factors that affect the nominal acoustical velocity that one might ordinarily rely upon to determine the corresponding bolt length for a given measurement. These factors include the effect on velocity resulting from the birefringent characteristics of a bolt under stress, the temperature under which the measurement is being made, and the coefficient of thermal expansion of the material being tested. Accordingly, these factors will detrimentally affect the accuracy of the length measurement unless means are provided for compensating for such factors. Unfortunately, prior art devices that rely upon the transmission of acoustic energy for time measurement which can then be related to the thickness of a material under test, do not provide means for compensating for such factors and would accordingly, yield an inaccurate result when applied to the particular measurement function described above. In addition to these disadvantages of the prior art, it is also to be noted that such prior art devices are particularly adapted to the measurement of thicknesses in the range of a few hundreths of an inch to sixteen to twenty inches maximum. Accordingly, such devices are not readily adapted to measuring bolt lengths that extend to seven feet or even longer. In addition to these relative disadvantages of the prior art, such prior art devices are not readily adapted to provide the measurement resolution required to display length changes that may be as small as 0.0001 inches that may ordinarily be required to measure relatively small changes in bolt lengths induced by moderate stresses as compared to their unstressed configuration.
Irrespective of bolt size, because of their relatively complex shapes from the standpoint of acoustic echo characteristics, bolts are inherently difficult to accurately measure. For example, a typical threaded bolt has a multitude of surfaces which tend to produce spurious echos that increase the likelihood of measurement error. No known prior art utilizing acoustic measurement techniques has satisfactorily solved this problem.
The following U.S. Patents are deemed to be relevant to the present invention:
______________________________________ U.S. Pat. No. Inventor ______________________________________ 3,354,700 Schindler 3,427,868 Charbonnier et al 3,605,504 Kummer Jr. et al 3,688,565 Brech 3,759,090 McFaul et al 3,918,294 Makino et al 3,962,910 Spyridakis et al 3,985,022 Dileo et al 4,064,735 Hutchison et al 4,104,778 Vilet 4,104,779 Sigmund 4,106,176 Rice et al 4,163,310 Sigmund 4,179,786 Eshghy ______________________________________
Of the above-identified patents some are relevant because they disclose measuring apparatus that utilize ultrasonic pulses for measuring the thickness of a test piece which may or may not be an elongated bolt and some are relevant because they disclose elongation measurement devices that are especially adapated to be used for measuring the elongation of bolts. The most relevant of the above-identified patents shall now be discussed in more detail.
The U.S. Pat. No. to Charbonnier et al (3,427,868) discloses an ultrasonic device for measuring the thickness of objects. The time period between a plurality of echo pulses is measured using a counter and a pulse duration multiplication circuit that permits the use of a low performance counter for measurement of the duration of a pulse that is proportional to the time interval between respective echos. Unfortunately, the Charbonnier patent does not disclose a device that is useable with elongated bolts, that is, a device that can overcome the special problems associated with length and elongation measurements as previously described. By way of example, Charbonnier does not disclose means for making temperature and stress factor corrections to the velocity of acoustic energy propagation, nor does it disclose means for measuring the nominal and elongated lengths of bolts that are as long as seven (7) feet or more. Furthermore, this patent does not disclose means for overcoming the special problems associated with noise and other reflection anomalies that are unique to ultrasonic measurement of elongated structures such as very long bolts.
Another form of ultrasonic thickness measuring system which utilizes a counter for measuring the interval between a transmitted burst and a reflected signal is disclosed by Kumer Jr. et al in U.S. Pat. No. 3,605,504. However, Kumer Jr. et al also fails to disclose a system which can overcome all the special problems associated with measuring the nominal length and elongation of a very long bolt.
The U.S. Pat. No. to McFaul et al (3,759,090) discloses an analog-type ultrasonic extensometer for measuring the elongation of a bolt. A meter is used to indicate when the time interval between the transmitted signal and the echo pulse from the far end of the bolt equals a preselected period of time which corresponds to a predetermined bolt length including bolt elongation due to stress. Although McFaul et al recognize the effects of temperature and stress on the accuracy of such measurements, they fail to disclose a convenient and accurate way of adjusting the measuring device to compensate for temperature and fail to disclose any means for adjusting for the effects of the velocity of acoustical energy in a stress bolt as compared to the velocity in an unstressed bolt by dismissing that change in velocity as being negligible and imperceptible in the meter readings of their device.
The U.S. Pat. No. to Makino et al (3,918,294) discloses a method for measuring the actual forces existing in bolts by applying an ultrasonic wave to the article under test to generate forced oscillations within the article, measuring a first natural frequency before the application of axial force, and measuring a second natural frequency subsequent to the application of axial force. Makino et al then determine the difference between those two frequencies and compare that difference against calibration data to determine the axial force during an actual test.
The U.S. Pat. No. to Dileo et al (3,985,022) discloses an ultrasonic measuring device which employs digital counting means and which provides an output indication of thickness of a work piece when the accumulated counts provided during successive time intervals of transmitted search pulses reaches a value that corresponds to a quantity of such search pulses which is commensurate with the acoustic velocity of the work piece. However, as with other prior art discussed previously, Dileo et al do not disclose means for accurately compensating for variations in acoustic velocity resulting from temperature and/or stress, nor do they disclose means for overcoming the special problems associated with measuring lengths of elongated bolts as previously described and as will be more fully understood hereinafter.